Motor actuator control system and method for controlling motor actuator

ABSTRACT

A motor actuator control system includes a motor actuator having a motor, an output shaft for outputting rotational motion of the motor at reduced rotational speed, and a gear train for transmitting the rotational motion to the output shaft while reducing rotational speed. The system further includes a first detector detecting a surge current generated at the moment that contact between a commutator and brushes in the motor is broken, a second detector detecting a predetermined rotational position of a gear, and a control unit controlling the motor on the basis of a signal from the first detector and another signal from the second detector. When the rotational position is detected, the position and the count of the surge current are correlated. Then, the motor is driven until the count reaches a predetermined number to rotate the output shaft to another predetermined rotational position.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is based on and incorporates herein by referenceJapanese Patent Application No. 2001-72327 filed on Mar. 14, 2001.

FIELD OF THE INVENTION

[0002] The present invention relates to a motor actuator control systemin which the number of rotations (rotational position shift amount) of amotor is detected to control the motor. The present invention ispreferably applied to, for example, an air conditioning system for avehicle in which a switching member such as a damper and an air mixingdoor of an air passage is driven by a motor actuator. In the airconditioning system, the position of the switching member is accuratelycontrolled by the motor actuator control system of the presentinvention.

BACKGROUND OF THE INVENTION

[0003] A switching member such as a damper and an air mixing door isused to switch the air flow mode between internal air circulation andexterior air introduction, to change air flow passages leading tointerior air outlet ports, and to control air mixing rate between hotair and cool air in an air conditioning system for a vehicle. Thoseactions in response to an operation of switches close to the driver'sseat are implemented by driving the switching member using a motoractuator. To ensure that the switching member is moved to apredetermined position, the position and the position shift amount ofthe switching member needs to be detected, and a motor in the actuatorneeds to be controlled on the basis of the detected information.

[0004] A system using a surge current periodically generated in themotor is proposed to detect the position and the position shift amountof the switching member. In the motor, a commutator rotatedsynchronously with a rotor slides on and discontinuously contactsbrushes to pass an electric current to rotor coils, so the contactbetween the commutator and the brushes is periodically made and broken.A surge (commutator surge) current is generated at the moment that thecontact is broken, so the commutator surge current is also periodicallygenerated. Thus, the number of rotations of the motor (rotationalposition shift amount) is detected by counting that of the commutatorsurge current, and so is the position shift amount of the switchingmember.

[0005] The commutator surge current is generated a plurality of timesper one rotation of the rotor, so the detection based on a commutatorsurge current count is basically accurate. However, the commutator surgecurrent is so weak when the motor starts and stops rotating that theweak commutator surge current is not always detected. In addition, thecommutator surge current is generated only while the motor iselectrically powered, so the commutator surge current count becomesinaccurate if the rotor rotates by its own momentum or is rotated byunexpected force after the motor is switched off. Therefore, thedetection based on the commutator surge current count is not reliableenough.

SUMMARY OF THE INVENTION

[0006] The present invention has been made in view of the above aspectswith an object to provide a motor actuator control system in which thenumber of rotations (rotational position shift amount) of a motor isaccurately controlled by correlating a count of commutator surge currentand a predetermined rotational position of a gear in a gear trainconnected to the motor.

[0007] In the present invention, the motor actuator control systemincludes a motor actuator having a motor, an output shaft for outputtingrotational motion of the motor at reduced rotational speed, and a geartrain constituted of a plurality of gears to transmit rotational motionof the motor to the output shaft while reducing rotational speed. Thecontrol system further includes a first detector detecting a commutatorsurge current, a second detector detecting a predetermined rotationalposition of a gear, and a control unit controlling the motor on thebasis of a signal from the first detector and another signal from thesecond detector.

[0008] The commutator surge current is counted by the control unit. Whenthe second detector detects the predetermined position of the outputgear which is connected to the output shaft and rotated at the slowestspeed in the gear train, the count of the commutator surge current iscorrelated with the predetermined rotational position by substituting apredetermined number for the count. Then, the motor is driven until thecount reaches another predetermined number in order to rotate the outputshaft to another predetermined rotational position.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The above and other objects, features and advantages of thepresent invention will become more apparent from the following detaileddescription made with reference to the accompanying drawings.

[0010] In the drawings:

[0011]FIG. 1 is a plan view of a motor actuator according to anembodiment of the present invention;

[0012]FIG. 2 is a side view of a pickup used in the embodiment;

[0013]FIG. 3 is a schematic block diagram of a motor actuator controlsystem according to the embodiment;

[0014]FIG. 4 is a schematic circuit diagram of the motor actuatorcontrol system according to the embodiment;

[0015]FIG. 5 is a time chart showing the correlation between surge(commutator surge) current and compensational signal (conductioncurrent);

[0016]FIG. 6 is a flow chart showing a routine to control the motoractuator control system according to the embodiment;

[0017]FIG. 7 is a schematic view of an air conditioning system for avehicle to which the motor actuator control system according to theembodiment is applied;

[0018]FIG. 8 is a schematic circuit diagram of a motor actuator controlsystem according to the first modification of the embodiment;

[0019]FIG. 9 is a schematic block diagram of the motor actuator controlsystem according to the first modification;

[0020]FIG. 10 is a time chart showing the correlation between surge(commutator surge) current and compensational signal (conductioncurrent) according to the first modification;

[0021]FIG. 11 is a schematic circuit diagram of a motor actuator controlsystem according to the second modification of the embodiment;

[0022]FIG. 12 is a schematic block diagram of the motor actuator controlsystem according to the second modification;

[0023]FIG. 13 is a schematic circuit diagram of a motor actuator controlsystem according to the third modification of the embodiment;

[0024]FIG. 14 is a plan view showing a modified pulse plate (conductivepart, ring-shaped part) according to other modification of theembodiment; and

[0025]FIG. 15 is a plan view showing another modified pulse plate(conductive part, ring-shaped part) according to other modification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0026] The present invention will be described in detail with referenceto an embodiment and various modifications of the embodiment, in whichthe same reference numerals designate same or similar members.

[0027] (Embodiment)

[0028] As shown in FIG. 1, a motor actuator 12 has a housing 14constituted of a main case 16 and a lid (not illustrated). The case 16is approximately box-shaped and has an opening. The lid closes theopening to shield the case 16. The case 16 stores a DC motor 20 having arotary shaft 22. A worm gear 24 is coaxially attached to the end of theshaft 22. The worm gear 24 meshes with a worm wheel 26 disposed by thegear 24. The worm wheel 26 has a support shaft constituted of a bottomshaft and a lid shaft. The bottom shaft and the lid shaft are rotatablysupported by a pair of bearings (not illustrated) respectively formed onthe bottom of the case 16 and on the lid. A gear 28 is formed on thebottom shaft of the worm wheel 26 in a coaxial relation with the wormwheel 26. The gear 28 meshes with a gear 30 disposed by the gears 26,28. The gear 30 has a support shaft constituted of a bottom shaft and alid shaft. The bottom shaft and the lid shaft are rotatably supported bya pair of bearings (not illustrated) respectively formed on the bottomof the case 16 and on the lid. A gear 32 is formed on the bottom shaftof the gear 30 in a coaxial relation with the gear 30. The gear 32meshes with an output gear 34 disposed by the gear 32. The output gear34 has a bottom shaft and an output shaft 35. The bottom shaft and theoutput shaft 35 are rotatably supported by a pair of bearings (notillustrated) respectively formed on the bottom of the case 16 and in thelid. The output shaft 35 penetrates the lid of the case 14 to beconnected to a damper 90, 91, 92 or an air mixing door 96 in an airconditioning system 82 for a vehicle as shown FIG. 7.

[0029] The air conditioning system 82 has three motor actuators 12. Eachactuator 12 is electrically connected to and controlled by a controller60. The first motor actuator 12 is mechanically connected to the damper90 using a link 93. The damper 90 switches air flow path between a duct85 for introducing interior air and a duct 84 for introducing exteriorair. The second motor actuator 12 is mechanically connected to thedampers 91 and 92 using links 94. The damper 91 switches air flow pathbetween a duct 86 leading to a defroster and a duct 87 leading tointerior air outlet ports. The damper 92 switches air flow path betweena duct 88 leading to an air outlet port close to the instrument paneland a duct 89 leading to an air outlet port close to passengers' feet.The third motor actuator 12 is mechanically connected to the door 96using a link 95 for controlling air mixing rate between hot airgenerated by a heater core 97 and cool air.

[0030] When the motor 20 is driven and the shaft 22 is rotated, therotational motion of the shaft 22 is transmitted to the output gear 34through a gear train constituted of the worm gear 24, the worm wheel 26,and the gears 28, 30, and 32 while rotational speed is reduced. Theoutput shaft 35, which is connected to the link 93, 94, 95 convertingrotary movement of the output shaft 35 of the actuator 12 intoreciprocative movement of the damper 90, 91, 92 or the door 96, drives arelated switching member. For example, the motor actuator 12 used formoving the damper 90 closes either one of the ducts 84 and 85 to stopair flow. For the sake of brevity, explanation on this embodiment andfollowing modifications will be made referring only to the motoractuator 12 used for moving the damper 90.

[0031] A yoke 40 which doubles a housing for the motor 20 has a bearing42 supporting rotatably the shaft 22. The yoke 40 stores a rotor 44which is coaxially penetrated by the shaft 22 and rotated synchronouslywith the shaft 22. The rotor 44 is wound with a wire forming a coil 46.Magnetic fields are generated when electricity is passed through thecoil 46. The rotor 44 has a commutator 48 at one end in the direction ofthe rotation axis of the rotor 44. The commutator 48 is constituted of apair of electrodes which are electrically connected to the coil 46.Those electrodes of the commutator 48 are integrated with the rotor 44and the shaft 22 so as to face each other around the shaft 22. A pair ofbrushes 50 are disposed in the yoke 40. One brush 50 contacts oneelectrode of the commutator 48, and the other brush 50 contacts theother electrode. As shown in FIG. 3, those brushes are electricallyconnected to a battery 52 using a lead wire or the like. Electricity ispassed from one brush 50 to the other through one electrode, the coil46, and the other electrode.

[0032] A pair of permanent magnets 54 are disposed in the yoke 40. Thosemagnets 54 are attached to the inner surface of the yoke 40 so as toface the coil 46 around the rotation axis of the shaft 22. The yoke 40is made of a metal to provide a magnetic flux path. The rotor 44 isrotated due to an interaction between the magnetic field of the coil 46generated when electricity is passed through the coil 46 and themagnetic field of those magnets 54.

[0033] As shown in FIG. 3, the motor 20 is electrically connected to amotor control circuit 62 in an electronic control unit 60 of a motoractuator control system 10. The circuit 62 controls the electricitysupplied from the battery 52 to the motor 20. The motor 20 is alsoconnected to a commutator surge current detection circuit 64 in thecontrol unit 60, which monitors a current (motor current) passed throughthe motor 20.

[0034] In the motor 20, the commutator 48 rotated synchronously with therotor 44 discontinuously contacts and slides on the brushes 50 to passthe electric current to the coil 46, so the contact between thecommutator 48 and the brushes 50 is periodically made and broken. Thecommutator surge current is generated due to the self-induction of thecoil 46 at the moment that the contact is broken, so the motor currentpassed through the motor 20 increases at the moment.

[0035] The circuit 64 sends a signal (commutator surge signal) to thecircuit 62 when the circuit 64 detects the commutator surge current,which is larger than a predetermined threshold intensity. The circuit 62controls the motor 20 on the basis of the count of the commutator surgesignal.

[0036] The contact between the commutator 48 and the brushes 50 is madeand broken due to the rotation of the commutator 48. Therefore, thecount of the commutator surge current is correlated to that of rotationof the motor 20 (shaft 22) and to the rotational position (positionshift ammount) of the output shaft 35 driven by the motor 20. The countis also correlated to the position (position shift amount) of the damper90. Thus, the position (position shift amount) of the output shaft 35(the damper 90) is accurately controlled by counting accurately thecommutator surge current.

[0037] As shown in FIG. 1, the output gear 34 has a pulse plate 66 onthe surface facing the lid of the housing 14. The pulse plate 66 is madeof an electrically conductive material such as metal, is in the shape ofa ring, and is attached to the output gear 34 in a coaxial relation. Apickup 68 is disposed on the same side of the output gear as the plate66 is formed. The pickup 68 is a resilient thin stick-like plate made ofan electrically conductive material such as metal. One end of the pickup68 is fixed to a plate 70 disposed in the proximity of the circumferenceof the output gear 34. The plate 70 is supported by either of the maincase 14 or the lid of the housing 14. The other end of the pickup 68 hasa V-shaped sliding part 72 at which the pickup 68 continuously contactsthe pulse plate 66, as shown in FIG. 2. In this embodiment, the part 72is V-shaped. However, other shapes such as U-shape may be used as well.

[0038] Beside the pickup 68, a pickup 74 is disposed. The pickup 74 hassubstantially the same structure as the pickup 68. A V-shaped slidingpart 72 of the pickup 74 is placed outside of the pulse plate 66 so asnot to contact the plate 66. A projection part 76 protruding outwardlyfrom the pulse plate 66 is formed on the output gear 34. The projectionpart 76 is made of the same electrically conductive material as used forthe pulse plate 66. The pulse plate 66 and the projection part 76 arefixed to the output gear 34 to be rotated synchronously with the gear34. The sliding part 72 of the pickup 74 is disposed on the track of theprojection part 76. Therefore, those pickups 68, 74, the pulse plate 66,and the projection part 76 constitute an electrical switch. The switchis turned on when the motor 20 is driven and the projection part 76meets the sliding part 72 of the pickup 74 at a predetermined rotationalposition of the output gear 34. A current (conduction current) due tothe conduction of the switch is detected by a conduction detectioncircuit 80 in the control unit 60, as shown in FIG. 3.

[0039] In this embodiment, the switch is formed on the output gear 34which is rotated at the slowest speed in the gear train, so the slidingpart 72 has the least sliding distance and the smallest abrasion.Therefore, the durability of the part 72 is improved. However, theswitch can be formed on other gears.

[0040] The pickup 68 is connected to the positive pole of the battery52. The pickup 74 is connected to a resistor 78, and the resistor 78 isconnected to the conduction detection circuit 80, as shown in FIGS. 3and 4, so the conduction current is output separately from the currentpassed through the motor 20 when the switch is turned on. The circuit 80is electrically connected to the motor control circuit 62 to send asignal (conduction signal) caused by the conduction current.

[0041] The motor control circuit 62 controls the motor 20 on the basisof a control routine shown in FIG. 6. After the motor actuator controlsystem 10 is set to function at step 200, whether an operation signalfrom an operation switch (not illustrated) is received or not isdetermined at step 202. If not, step 202 is repeated awaiting theoperation signal. If the operation signal is detected at step 202, acount N of the commutator surge current is reset by substituting zerofor the count N at step 204. Then, the commutator surge currentdetection circuit 64 is set to monitor the motor current passed throughthe motor 20 at step 206. The conduction detection circuit 80 is set tomonitor the conduction current. Afterward, the motor 20 is driven atstep 210.

[0042] At step 212, whether the circuit 62 receives the conductionsignal or not is determined. If the conduction signal is received,whether the count N of the commutator surge current is equal to apredetermined count NA or not is determined at step 224.

[0043] The count N is bound to be equal to the predetermined count NA ifthe rotational position of the shaft 22 is at a predetermined positionwhen the motor 20 is turned on and the commutator surge currentgenerated right after the motor 20 is turned on have intensity strongenough the commutator surge current detection circuit 64 can detect. Ifthe count N is equal to the predetermined count NA, step 212 isrepeated.

[0044] The count N is not equal to the predetermined count NA if therotational position is shifted from the predetermined position due to anexpected external force or if the commutator surge current does not haveintensity strong enough. In the case that the count N is not equal tothe predetermined count NA, the count N is corrected by substituting thepredetermined count NA for the count N. Then, step 212 is repeated. Inthis correction, even if the real rotational position of the shaft 35 isshifted from a detected position based on the count N, the correlationbetween them is retrieved using the conduction signal. Therefore, thereliability in controlling the rotational position of the shaft 35 isimproved.

[0045] If the conduction signal is not received at 212, whether thecircuit 62 receives the commutator surge signal or not is determined atstep 214. As described above, the commutator surge signal is sent fromthe circuit 64 to the circuit 62 when the circuit 64 detects thecommutator surge current larger than a predetermined thresholdintensity. If the commutator surge signal is not received at step 214,step 212 is repeated. If received, one is added to the count N at step216, and then step 218 is executed. At step 218, whether the count Nreaches another predetermined count NS or not is determined. If not,step 212 is repeated. If the count N reaches the count NS, step 220 isexecuted to stop the motor 20, and then the routine is ended at step222.

[0046] In the motor actuator 12 used for moving the damper 90, the countNS is the count of the commutator surge current generated while thedamper 90 closing one of the ducts 84 and 85 is moved to the positionwhere the damper 90 closes the other. Therefore, the damper 90 is surelymoved to the position by turning off the motor 20 when the count Nreaches the predetermined count NS.

[0047] The commutator surge current is generated at the moment that thecontact between the commutator 48 and the brushes 50 is broken. Inaddition, the rotational motion of the shaft 22 is transmitted to theoutput shaft 35 with reduced rotational speed in the motor actuator 12.Therefore, the count of the commutator surge current generated while theoutput gear 34 spins once is the product of the inverse of the overallspeed reduction ratio and the number of electrodes constituting thecommutator 48. Thus, the motor 20 is precisely controlled on the basisof the count of the commutator surge current in the motor actuatorcontrol system 10.

[0048] (First Modification)

[0049] As shown in FIG. 8, the motor 20 and those pickups 68, 74 areconnected electrically in parallel in a motor actuator control system100. The pickup 68 is electrically connected to the positive pole of thebattery 50, to which one brush 50 is electrically connected, with aresistor 102 interposed between the pole and the pickup 68. The pickup74 is electrically connected to the negative pole of the battery 50 towhich the other brush 50 is electrically connected. In thismodification, the positive and the negative poles of the battery 52 arerespectively assigned to those pickups 68, 74. However, the oppositeassignment may be used.

[0050] As shown in FIG. 9, the motor actuator control system 100 has acontrol unit 104 constituted of the motor control circuit 62 and acommutator surge and conduction signal detection circuit 106. Thedetection circuit 106 monitors the motor current passed through themotor 20 and sends the commutator surge signal to the motor controlcircuit 62 in the control unit 104 when the circuit 106 detects thecommutator surge current. The detection circuit 106 also detects theconduction current passed between those pickups 68, 74 and sends theconduction signal to the motor control circuit 62 when the circuit 106detects the conduction current. The conduction current provides a muchhigher peak T than a peak due to the commutator surge current in themotor current, as shown in FIG. 10. The motor control circuit 62controls the motor 20 on the basis of the commutator surge signal andthe conduction signal in line with the control routine shown in FIG. 6as in the embodiment.

[0051] In this modification, the motor 20 and those pickups 68, 74 areconnected electrically in parallel in a motor actuator control system100, so a wiring harness connecting the pickup 74 to the conductiondetection circuit 80 in the embodiment is needless and the system 100becomes simpler in structure than the system 10.

[0052] (Second Modification)

[0053] As shown in FIGS. 11 and 12, the motor circuit including themotor 20 and another circuit including those pickups 68, 74 areseparately connected to the battery 52 in a motor actuator controlsystem 120. Therefore, the conduction current passed between thosepickups 68, 74 is not affected by a fluctuation of the motor current.Thus, the conduction current is more preferably detected by theconduction detection circuit 80.

[0054] (Third Modification)

[0055] As shown in FIG. 13, a resistor 142 is disposed between thepickup 68 and the positive pole of the battery 52, which is connected toone brush 50 in a motor actuator control system 140. Another resistor144 is disposed between the pickup 68 and the negative pole of thebattery 52, which is connected to the other brush 50. The pickup 74 isconnected to the conduction detection circuit 80 as in the system 10 inthe embodiment.

[0056] In the case that those resistors 142, 144 have the sameresistance, the circuit 80 incurs half the voltage of the battery 52when the conduction between those pickups 68, 74 is made. This circuitstructure also enables the conduction detection circuit 80 to detectpreferably the conduction current between those pickups 68, 74.

[0057] (Other Modifications)

[0058] In above embodiment and modifications, a single projection part76 is formed. However, a plurality of projection parts 76 may be formedwith a constant angular interval X1, as shown in FIG. 14. Eightprojection parts are formed with forty-five degree angular interval inFIG. 14. In this case, at least one conduction between those pickups 68,74 is made, namely at least one conduction signal is sent from theconduction detection circuit 80 to the motor control circuit 62 when theoutput gear 34 is rotated by forty-five degrees or more.

[0059] In the case that the circuit 62 is programmed to cancel a secondand later conduction signals, a motor actuator control system in thismodification performs in the same manner as the system 10 in theembodiment. In the case that the circuit 62 is programmed to correct thecount N using a plurality of predetermined counts NA every time theconduction signal is detected, the motor 20 is more accuratelycontrolled due to the multiple correction.

[0060] In above embodiment and modifications, each projection part 76protrudes outwardly from the pulse plate 66. However, each projectionpart 76 may protrudes inwardly from the pulse plate 66. In FIG. 15, aplurality of projection parts 76 protruding inwardly from the pulseplate 66 are formed with a constant angular interval X2. Twelveprojection parts are formed with thirty degree angular interval in FIG.15.

[0061] The constant angular interval X1, X2 is set to be smaller than apredetermined operation angle of the output shaft 35 formed on theoutput gear 34 in order to detect surely the rotational position of theoutput gear 34 within the operation angle. For example, in the case thatthe predetermined angle is sixty degrees, at least seven projectionparts 76 are needed to provide the constant angular interval X1, X2smaller than the operation angle.

[0062] In the case that only one projection part 76 is formed on theoutput gear 34, the position of the gear 34 in the rotational directionneeds to be adjusted such that the projection part 76 contacts thepickup 74 within the operation angle of the output shaft 35 when theoutput gear 34 is assembled. However, if a plurality of projection parts76 are formed with the constant angular interval X1, X2 smaller than theoperation angle, the position adjustment is needless. Therefore, theassembly becomes easier.

[0063] In the above embodiment and modifications, the rotationalposition of the output gear 34 is correlated with the count N of thecommutator surge current by substituting a predetermined number NA forthe count N unless the count N and the number NA are equal at the momentthat the motor control circuit 62 receives the conduction signal fromthe conduction detection circuit 80. However, the rotational position ofthe output gear 34 may be correlated with the count N of the commutatorsurge current by starting to count the commutator surge current at themoment that the circuit 62 receives the conduction signal from thecircuit 80.

[0064] In the above embodiment and modifications, the rotationalposition of the gear 34 is detected using the electric signal generatedby a mechanical switch constituted of those pickups 68, 74, the pulseplate 66, and the projection part 76. However, instead of the mechanicalswitch, other means such as an optical sensor and a magnetic sensor maybe used.

[0065] In the above embodiment and modifications, the motor actuatorcontrol system 10, 100, 120, 140 according the present invention is usedin the air conditioning system 82. However, as a matter of course, thesystem is not limited to the application and may be applied to othersystems in which at least one motor actuator is used.

What is claimed is:
 1. A motor actuator control system comprising: amotor having a commutator and a pair of brushes so that an electriccurrent is passed to the commutator through those brushes; an outputshaft for outputting rotational motion of the motor at reducedrotational speed; a gear train constituted of a plurality of gears fortransmitting rotational motion of the motor to the output shaft whilereducing rotational speed; a motor rotational position detector fordetecting a surge current generated at a moment that contact between thecommutator and the brushes is broken; an output shaft rotationalposition detector for detecting rotational position of a gear located inan output side of an input gear driven directly by the motor; and acontrol unit for controlling the motor on a basis of the rotationalposition and a count of the surge current.
 2. The system as in claim 1,wherein the position-detected gear is an output gear which is connectedto the output shaft and rotated at the slowest speed in the gear train.3. The system as in claim 1, wherein the output shaft rotationalposition detector includes a switch constituted of: an electricallyconductive member formed on the position-detected gear so as to berotated synchronously with the position-detected gear; and first andsecond pickups associated with the conductive member in a way that bothpickups contact the conductive member to turn on the switch only whenthe position-detected gear comes to a predetermined rotational position.4. The system as in claim 3, wherein: the first pickup is in acontinuous contact with the conductive member; and the second pickup isin a contact with the conductive member to turn on the switch only whenthe position-detected gear comes to a predetermined rotational position.5. The system as in claim 4, wherein the conductive member includes: aring-shaped part which is in a concentric relation with theposition-detected gear and in a continuous contact with the firstpickup; and a projection part which protrudes from the ring-shaped partin a radial direction of the ring-shaped part and is in a contact withthe second pickup to turn on the switch only when the position-detectedgear comes to a predetermined rotational position.
 6. The system as inclaim 5, wherein: the output shaft is rotatable within a predeterminedangle to drive a member used to change air passages in an airconditioning system; and a plurality of projection parts are formed suchthat a maximum angular interval between the projection parts is smallerthan the predetermined angle.
 7. The system as in claim 6, wherein thepredetermined angle is sixty degrees.
 8. A method for controlling amotor actuator including a motor having a commutator and a pair ofbrushes, an output shaft for outputting rotational motion of the motorat reduced rotational speed, and a gear train constituted of a pluralityof gears to transmit rotational motion of the motor to the output shaftwhile reducing rotational speed, the method comprising steps of:detecting a predetermined rotational position of a gear located in anoutput side of an input gear driven directly by the motor in the geartrain; detecting a surge current generated at a moment that contactbetween the commutator and those brushes is broken; correlating theposition and a count of the surge current; comparing the count with apredetermined number; and stopping the motor if the count reaches thepredetermined number.
 9. The method as in claim 8, wherein the step ofcorrelating is made by starting to count the surge current at a momentthat the predetermined rotational position is detected.
 10. The methodas in claim 8, wherein the step of correlating is made by substitutinganother predetermined number for the count at a moment that thepredetermined rotational position is detected.
 11. The method as inclaim 8, wherein the step of detecting the predetermined rotationalposition is of an output gear which is connected to the output shaft androtated at the slowest speed in the gear train.
 12. The method as inclaim 8, wherein the step of detecting the predetermined rotationalposition is done using an electric signal generated by a switchconstituted of: an electrically conductive member formed on theposition-detected gear so as to be rotated synchronously with theposition-detected gear; and first and second pickups associated with theconductive member in a way that both pickups contact the conductivemember to turn on the switch only when the position-detected gear comesto a predetermined rotational position.
 13. The method as in claim 12,wherein: the first pickup continuously contacts the conductive member;and the second pickup contacts the conductive member to turn on theswitch only when the position-detected gear comes to a predeterminedrotational position.
 14. The method as in claim 13, wherein: the firstpickup continuously contacts the conductive member at a ring-shaped partof the conductive member which is disposed in a concentric relation withthe position-detected gear; and the second pickup contacts a projectionpart of the conductive member which protrudes from the ring-shaped partin a radial direction of the ring-shaped part.
 15. The method as inclaim 14, wherein: the output shaft is rotated within a predeterminedangle to drive a member used to change air passages in an airconditioning system; and the predetermined position is detected usingany of a plurality of projection parts formed such that a maximumangular interval between the projection parts is smaller than thepredetermined angle.
 16. The method as in claim 15, wherein thepredetermined position is detected using any of a plurality ofprojection parts having sixty degree angular interval.